The present invention relates to an arrangement in contact drying of a paper web, wherein the paper web is dried on heated smooth-faced drying cylinders by means of a number of successive so-called normal groups with single-wire draw, in which the drying cylinders are placed in an upper row and reversing suction cylinders or equivalent reversing suction rolls are placed in the lower row. After the press section of the paper machine, the paper web is dried initially in a number of successive drying groups with single-wire draw by pressing the paper web by means of the drying wires of the drying groups against the heated faces of the drying cylinders and, in each group with single-wire draw, by guiding the paper web on support of the same drying wire from one drying cylinder onto the next drying cylinder over the reversing suction cylinders or rolls. When the paper web is placed on the drying wire at the side of the outside curve, the web is kept on the drying wires by means of a difference in pressure against the effect of centrifugal forces.
The invention also relates to a dryer section of a paper machine, which is composed of a number of successive so-called normal groups with single-wire draw, from the start thereof and over the major part of its length, in which the drying cylinders are placed in the upper row and the reversing suction cylinders or corresponding suction rolls are placed in the lower row. Between the normal groups, the paper web to be dried has closed draws over the group gaps. The reversing suction cylinders or corresponding suction rolls are arranged to be subjected to a vacuum, at least in respect of their turning sectors of the drying wire.
In the manner known from the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible of fluttering and may cause web breaks, in particular since the web is still relatively moist and, therefore, has a low strength. For this reason, in recent years, increasing use has been made of a single-wire draw, in which each group of drying cylinders has one drying wire only, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas, on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the drying wire loop and the reversing cylinders or rolls are placed inside the loop.
In dryer sections that comprise inverted and normal drying groups, various problems have occurred, for which problems the present invention suggests novel, efficient solutions. These problems include the large length of the dryer section which increases the costs of the dryer section and of the machine hall. Problems have also been encountered in the runnability of the dryer section and in the threading of the web, problems arising from differences in the speeds of different wires, as well as problems related to the control of transverse shrinkage of the web.
In inverted drying groups, in the event of web breaks, a further problem consists of the removal of broke, for inverted groups are not self-cleaning by the force of gravity. Generally, these problems tend to become worse as the running speed of the paper machine becomes higher.
In prior art dryer sections in which exclusively groups with single-wire draw are used, in the last wire groups, considerable wear of the drying fabrics has occurred, in particular in the manufacture of fine papers with a high content of fillers. Thus, the present invention is also directed to reducing this problem which occurs in the drying groups driven by the drying wire because of the considerable thickness of the drying wire. The differences in speed mentioned above, together with a restricted drying shrinkage, have caused web breaks in the last groups when exclusively single-wire draw has been used. This problem is emphasized further if, in the groups with single-wire draw, small-diameter suction rolls proper are used that are provided with an inside suction box. In order to eliminate this problem, in some machines, it has been even necessary to open some group gaps and to lower the level of negative pressure in the suction rolls. Problems similar or corresponding to those described above are encountered, e.g., in the dryer section described in the U.S. Pat. No. 5,269,074 (assigned to Beloit Corp.), wherein in the groups with single-wire draw, normal suction rolls are used which have very small diameters and which are provided with inside suction boxes. Moreover, in this patent, a dryer section is described in which the last group is a single group with twin-wire draw, in which there are two rows of drying cylinders placed one row above the other, the web having free draws between these rows. A dryer section similar to that mentioned above is also described in the assignee's Finnish Patent Application No. 934367 (filed on Oct. 5, 1993 and corresponding to U.S. patent application Ser. No. 08/213,148, the specification of which is hereby incorporated by reference herein).
In the above-mentioned U.S. patent and Finnish patent application, it has been the starting point that the tensile strength of a paper web is increased substantially when its dry solids content becomes higher, so that it has been considered that the group with twin-wire draw should be placed preferably expressly as the last group in whose area the dry solids content of the web is at the maximum, and so is the tensile strength at the maximum in view of elimination of the problems of runnability produced by the free draws of the web. Then, consideration has, however, not been given to the fact that the susceptibility of breaks at the free draws of the web does not depend on the tensile strength of the web alone, but also on the breaking strength (tensile-energy absorption) and the tear strength in particular in respect of breaks and wrinkles starting in the lateral areas of the web.
With respect to the different strength properties of the paper web, reference is made in this connection to the paper by Lars Nordman, Jan-Erik Levlin and Jukka Visti entitled "Kuivatuksen vaikutus paperirainan ominaisuuksiin", Paperin valmistus, Suomen paperi insinoorien oppi- ja kasikirja III, osa 1; editor A. Arjas, Teknillisten tieteiden Akatemia, Turku 1982, pages 705-724.